High tetragonality barium titanate-based compositions and methods of forming the same

ABSTRACT

Barium titanate-based compositions having a high tetragonality and methods of forming the same are provided, as well as devices formed from the compositions. The barium titanate-based compositions advantageously have a high tetragonality and small particle sizes. For example, in some embodiments, the barium titanate-based compositions have a tetragonality of greater than about 2.0 and an average particle size of less than about 0.3 micron. Some methods involve achieving high tetragonality by limiting the concentration of certain metals (other than barium or titanium) in the compositions and/or heat treating the compositions at relatively high temperatures. In some methods, the A/B ratio of the composition may be adjusted prior to heat treatment to ensure that a small particle size is maintained during heat treatment. The barium titanate-based compositions may be processed to form dielectric layers in electronic devices, such as MLCCs, having excellent electrical properties as a result of the high tetragonality and relatively small particle sizes.

FIELD OF INVENTION

[0001] The invention relates generally to dielectric materials and, moreparticularly, to barium titanate-based compositions having a hightetragonality and methods of forming the same.

BACKGROUND OF INVENTION

[0002] Barium titanate-based materials, which include barium titanate(BaTiO₃) and its solid solutions, may be used as dielectric materials inelectronic devices. For example, barium titanate-based particulatecompositions may be processed to form dielectric layers in multilayerceramic capacitors (MLCCs). Barium titanate-based materials are used inMLCC devices because barium titanate can have a high dielectricconstant. The high dielectric constant arises, in part, because bariumtitanate can assume a tetragonal perovskite crystal structure at roomtemperature.

[0003] Unit cells of crystal structures have dimensions a, b and c whichcorrespond to the x, y and z axes of the cell. In tetragonal unit cells,a=b≠c. The “tetragonality” of a tetragonal unit cell is related to theratio of c/a. Tetragonality may be measured using standard x-raydiffraction techniques. There are a number of factors that can effectthe tetragonality of barium titanate-based materials including particlesize. Generally, small particles (for example, less than about 0.5micron) have low tetragonalities, and tetragonality increases asparticle size increases.

[0004] As the tetragonality of a barium titanate-based materialincreases, its dielectric constant increases for a given particle size.Therefore, it is desirable to use barium titanate-based materials havinga high tetragonality in MLCC applications. It is also desirable to usesmall particles (for example, less than about 0.5 micron) to formdielectric layers in MLCC applications in order to reduce dielectriclayer thickness. Thinner dielectric layers allow an MLCC manufacturer toincrease the number of dielectric layers in an MLCC of a given thicknesswhich improves certain device characteristics. However, the dielectricconstant of MLCCs formed using small barium titanate-based particles maybe sacrificed because of the relatively low tetragonality of suchparticles.

SUMMARY OF INVENTION

[0005] The invention provides barium titanate-based compositions havinga high tetragonality and methods of forming the same, as well as devicesformed from the compositions.

[0006] In one aspect, the invention provides a composition comprisingbarium titanate-based particles having an average particle size of equalto or less than about 0.3 micron and a tetragonality of equal to orgreater than about 2.0.

[0007] In another aspect, the invention provides a compositioncomprising barium titanate-based particles having an average particlesize of equal to or less than about 0.15 micron and a tetragonality ofequal to or greater than about 1.5.

[0008] In another aspect, the invention provides a multilayer ceramiccapacitor. The multilayer ceramic capacitor comprises an electrodelayer; and a dielectric layer formed on the electrode layer. Thedielectric layer includes a plurality of grains, the grains comprising abarium titanate-based material and having an average grain size of equalto or less than about 0.3 micron and a tetragonality of equal to orgreater than about 2.0.

[0009] In another aspect, the invention provides a method. The methodcomprises mixing a barium source and a titanium source to form areaction mixture. The barium source has a concentration of strontium andcalcium of less than about 200 ppm. The method further comprisesmaintaining the reaction mixture at an elevated temperature while thebarium source reacts with the titanium source to produce bariumtitanate-based particles; and, heat treating the barium titanate-basedparticles at a temperature of between about 850° C. and about 1150° C.

[0010] In another aspect, the invention provides a method. The methodcomprises mixing a barium source and a titanium source to form areaction mixture, and maintaining the reaction mixture at an elevatedtemperature while the barium source reacts with the titanium source toproduce barium titanate-based particles. The method further comprisesadjusting an A/B ratio of the barium titanate-based particles; and, heattreating the barium titanate-based particles, after A/B ratioadjustment, at a temperature of between about 850° C. and about 1150° C.

[0011] Other aspects, embodiments, and features of the invention willbecome apparent from the following detailed description. All referencesincorporated herein are incorporated in their entirety. In cases ofconflict between an incorporated reference and the presentspecification, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIGS. 1A-1C respectively show schematic x-ray diffraction patternof a barium titanate-based composition.

[0013]FIG. 2 is a graph showing the effect of heat treatment temperatureon tetragonality for samples produced in Example 1.

[0014]FIG. 3 is a graph showing the effect of strontium concentration ontetragonality for samples produced in Example 1.

DETAILED DESCRIPTION

[0015] Barium titanate-based compositions having a high tetragonalityand methods of forming the same are provided, as well as devices formedfrom the compositions. The barium titanate-based compositionsadvantageously have a high tetragonality and small particle sizes. Forexample, in some embodiments, the barium titanate-based compositionshave a tetragonality of equal to or greater than about 2.0. and anaverage particle size of equal to or less than about 0.3 micron. Asdescribed further below, some methods involve achieving hightetragonality by limiting the concentration of certain metals (otherthan barium or titanium) in the compositions and/or heat treating thecompositions at relatively high temperatures. In some methods, the A/Bratio of the composition may be adjusted prior to heat treatment toensure that a small particle size is maintained during heat treatment.The barium titanate-based compositions may be processed to formdielectric layers in electronic devices, such as MLCCs, having excellentelectrical properties as a result of the high tetragonality andrelatively small particle sizes.

[0016] As used herein, the “tetragonality” of the barium titanate-basedcomposition refers to the ratio (I₂₀₀/I_(b)). The ratio may bedetermined from an x-ray diffraction pattern that plots Intensity(counts) versus 2-Theta (degrees). I₂₀₀ is the value of the intensity atthe (200) peak on an x-ray diffraction pattern obtained from thecomposition. When a clear valley exists between the (002) and (200)peaks on the x-ray diffraction pattern (FIG. 1A), I_(b) is defined asthe intensity at the minima of the valley defined as d(I)/d(2-Theta)=0.A clear valley exists when d(I)/d(2-Theta) has a minimum of less than 0.When a clear valley does not exist between the (002) and (200) peaks onthe x-ray diffraction pattern (FIG. 1B), I_(b) is defined as theintensity where d(I)/d(2-Theta) has a minimum value of greater than 45degrees and less than the I₂₀₀ peak. The x-ray diffraction pattern maybe obtained using standard x-ray powder diffraction techniques. Onesuitable technique involves using a copper target at a voltage of 40 kVand a current of 35 mA to provide a copper K-alpha monochromatic x-raysource, a sample rotation speed of 60 rpm, a scan rate of 0.1°/minuteand a scan range of 44°-46° 2-Theta. FIGS. 1A-1C show schematic x-raydiffraction patterns including I₂₀₀ and I_(b).

[0017] As described further below, methods of the present invention maybe used to produce barium titanate-based compositions having a desirablyhigh tetragonality. Generally, the barium titanate-based compositionshave a tetragonality of equal to or greater than about 1.5. In somecases, the compositions have a tetragonality of equal to or greater thanabout 2.0. or equal to or greater than about 2.5. In some cases, when avery high tetragonality is desired, the compositions may have atetragonality of equal to or greater than about 3.0. The desiredtetragonality of the barium titanate-based composition depends, in part,on the application and performance requirements in which the compositionis used.

[0018] As used herein, “barium titanate-based” compositions refer tobarium titanate, solid solutions thereof, or other oxides based onbarium and titanium having the general structure ABO₃, where Arepresents one or more divalent metals such as barium, calcium, lead,strontium, magnesium and zinc and B represents one or more tetravalentmetals such as titanium, tin, zirconium and hafnium. One type of bariumtitanate-based composition has the structureBa_((1-x))A_(x)Ti_((1-y))B_(y)O₃, where x and y can be in the range of 0to 1, where A represents one or more divalent metal other than bariumsuch as lead, calcium, strontium, magnesium and zinc and B representsone or more tetravalent metals other than titanium such as tin,zirconium and hafnium. Where the divalent or tetravalent metals arepresent as impurities, the value of x and y may be small, for exampleless than 0.1. In other cases, the divalent or tetravalent metals may beintroduced at higher levels to provide a significantly identifiablecompound such as barium-calcium titanate, barium-strontium titanate,barium titanate-zirconate and the like. In still other cases, where x ory is 1.0, barium or titanium may be completely replaced by thealternative metal of appropriate valence to provide a compound such aslead titanate or barium zirconate. In other cases, the compound may havemultiple partial substitutions of barium or titanium. An example of sucha multiple partial substituted composition is represented by thestructural formulaBa_((1-x-x′-x″))Pb_(x)Ca_(x′)Sr_(x″)O.Ti_((1-y-y′-y″))Sn_(y)Zr_(y′)Hf_(y″)O₂,where x, x′, x″, y, y′, and y″ are each greater than or equal to 0. Inmany cases, the barium titanate-based material will have a perovskitecrystal structure, though in other cases it may not.

[0019] Barium titanate (i.e., BaTiO₃) particles may be preferred in someembodiments of the invention. In particular, relatively pure bariumtitanate particles may be preferred because of their high tetragonality.It has been discovered that limiting concentrations of certaincontaminant metals (other than barium or titanium) in the bariumtitanate particle composition can increase the tetragonality of thecomposition. It is believed that the presence of such contaminants cansubstitute on barium or titanium lattice sites and, thus, disrupt theformation of tetragonal unit cells. For example, as described furtherbelow, limiting the presence of divalent metals other than barium, suchas strontium and/or calcium, which are common contaminants in bariumtitanate particle compositions may increase tetragonality. In somecases, the barium titanate particles have less than 500 parts permillion of strontium and/or calcium. In other cases, the barium titanateparticles have less than 200 parts per million, or even less than 50parts per million, of strontium and/or calcium. In some cases, thecalcium and/or strontium concentration may be substantially zero.Techniques for limiting strontium and/or calcium concentration aredescribed below.

[0020] However, it should also be understood that not all particlesproduced in accordance with the invention have low contamination levels(i.e., less than 500 parts per million) when other techniques are usedto increase the tetragonality of the composition to the desired level.

[0021] The barium titanate-based particles may have a variety ofdifferent particle characteristics. As noted above, the bariumtitanate-based particles typically have a small particle sizes, such asan average particle size of equal to or less than about 0.5 micron. Asused herein, the average particle size refers to the average size of theprimary particles of the composition. The average particle size of acomposition may be determined using SEM image analysis by measuring thesize of a representative number of particles (e.g., 300). In some cases,the average particle size is equal to or less than about 0.30 micron; insome cases, the average particle size may be equal to or less than about0.20 micron; in some cases, the average particle size is equal to orless than about 0.15 micron; and, in some cases, the average particlesize is equal to or less than about 0.10 micron. Particle size may becontrolled by the processing technique used and processing conditions,as described further below. The desired particle size depends, in part,on the application and desired characteristics of the resulting device.

[0022] As noted above, compositions of the present invention may havehigh tetragonalities and small particle sizes. Accordingly, thecompositions may have any of the above-described tetragonalities coupledwith any of the above-described particle sizes. For example, thecompositions may have a particle size of equal to or less than about 0.3micron and a tetragonality of equal to or greater than about 2.0, 2.5,or 3.0. The compositions may also have a particle size of equal to orless than about 0.2 micron and a tetragonality of equal to or greaterthan about 2.0, 2.5, or 3.0. The compositions may also have a particlesize of equal to or less than about 0.15 micron and a tetragonality ofequal to or greater than about 1.5, 2.0, 2.5, or 3.0. The particularcombination of particle size and tetragonality of the composition may beselected based on the particular application and achieved using themethods described herein.

[0023] The barium titanate-based particles may have a variety of shapeswhich may depend, in part, upon the process used to produce theparticles. The barium titanate-based particles may be equiaxed and/orsubstantially spherical, in particular, if the particles arehydrothermally produced as described further below. In some cases, theparticles may have an irregular, non-equiaxed shape.

[0024] The barium titanate-based particles may be produced in ahydrothermal process. Hydrothermal processes generally involve mixing abarium source with a titanium source in an aqueous environment to form ahydrothermal reaction mixture which is maintained at an elevatedtemperature. A suitable barium source is barium hydroxide solution whichmay be heated to an elevated temperature. A suitable titanium source isa hydrous titania gel. The titania gel may be produced by mixingtitanium oxychloride (TiOCl₂) with water, and then adding ammoniahydroxide to increase the pH of the solution, thus, precipitating thetitania gel. The titania gel may be washed to remove excess chloride anddispersed in water.

[0025] In the reaction mixture, the barium source reacts with thetitanium source to produce barium titanate particles which remaindispersed in the aqueous environment to form a slurry. The particles maybe washed to remove excess barium ions from the hydrothermal processwhile being maintained in the slurry. Suitable hydrothermal processesfor forming barium titanate-based particles have been described, forexample, in commonly-owned U.S. Pat. Nos. 4,829,033, 4,832,939, and4,863,883, which are incorporated herein by reference in theirentireties. Hydrothermal processes are particularly well-suited toproduce particles having a small average particle size (e.g., equal toor less than 0.5 micron) and/or a substantially spherical shape.Particle size may be controlled, for example, by adjusting thehydrothermal reaction conditions such as reaction time.

[0026] It should also be understood that the particles may be producedaccording to other suitable techniques known in the art includingsolid-state reaction processes, sol-gel processes, as well asprecipitation and subsequent calcination processes, such asoxalate-based processes.

[0027] As noted above, limiting the presence of metals other than bariumor titanium (e.g., strontium and/or calcium) in barium titanate particlecompositions may increase the tetragonality of the compositions.Contaminants (e.g., strontium and/or calcium) can be introduced intohydrothermally-produced barium titanate because such contaminants may bepresent in the barium source. Some hydrothermal methods of the presentinvention use barium sources that have low concentrations of contaminantmetals, such as strontium and/or calcium, to limit the presence of suchmetals in the resulting composition. For example, in some cases, sourceshaving a concentration of less than about 500 parts per million ofstrontium and/or calcium are used. In other cases, sources having aconcentration of less than about 200 parts per million, or even lessthan about 50 parts per million, of strontium and/or calcium are used.

[0028] The strontium and/or calcium concentration in the barium sourcedepends, in part, upon the raw material source. The methods of thepresent invention typically involve selecting barium sources having thedesired contaminant metal concentrations. However, it should beunderstood that purification techniques may also be used to reduce thecontaminant metal concentration to the desired level.

[0029] In some embodiments, methods of the invention subject the bariumtitanate-based particles to a heat treatment step to increase thetetragonality of the composition. For example, the particles may beheated to a temperature of between about 850° C. and about 1150° C. toincrease the tetragonality to the values described. In some cases, thetemperature is between about 950° C. and about 1050° C. Generally,tetragonality increases with increasing temperature within these ranges.Different temperatures may be used to achieve different tetragonalities.However, heat treatment temperatures of greater than about 1150° C. aretypically unsuitable because the particles begin to sinter at suchtemperatures.

[0030] When hydrothermally-produced barium titanate-based particles aresubjected to a heat treatment step, the water in the slurry may beremoved (e.g., by filtering or decanting) and the particles may be driedat a lower temperature prior to heat treatment. General heat treatmentprocesses have been described in commonly-owned, co-pending U.S. patentapplication Ser. No. 09/689,093, which was filed on Sep. 12, 2000, andis incorporated herein by reference in its entirety.

[0031] It should be understood that the tetragonality of the compositionmay be increased even when the heat treatment step does notsubstantially, if at all, increase the average particle size of thecomposition. In some embodiments, the A/B ratio of the bariumtitanate-based composition may be adjusted prior to the step of heattreating the coated particles in order to reduce the amount of particlegrowth during heat treatment. As used herein, A/B ratio is defined asthe ratio of divalent metals (e.g., Ba.) to tetravalent metals (e.g.,Ti) in the barium titanate-based particle composition. The A/B ratio maybe measured by determining the concentration of divalent metals andtetravalent metals in a composition using an XRF compositionalmeasurement technique.

[0032] It has been discovered, for example, that the amount of particlegrowth during heat treatment may be suitably controlled by adjusting theA/B ratio to a value other than 1.000. For example, the A/B ratio may beadjusted to values below 1.000, such as between about 0.975 and about1.000, or between about 0.982 and about 0.988. The A/B ratio may also beadjusted to values above 1.000, such as between about 1.000 and about1.025, or between about 1.008 and about 1.012. In certain MLCCapplications, it may be preferred to adjust the A/B ratio to a value ofgreater than 1.000 to increase the compatibility of the composition withbase metal electrodes.

[0033] The A/B ratio may be adjusted using any suitable technique. TheA/B ratio may be adjusted to values of greater than 1.000 by adding acompound comprising an A group element. In some embodiments, thecompound comprising an A group element (e.g., BaCO₃ or BaSiO₃) is coatedon the barium titanate-based particles using a precipitation techniquedescribed further below. In other embodiments, the A group elementcompound may be added in particulate form to the composition. The A/Bratio may be adjusted to values of less than 1.000 by washing thecomposition with a fluid (e.g., water) in which a small percentage ofthe A group element compound is dissolved.

[0034] In some methods of the present invention, the bariumtitanate-based particles may be deagglomerated prior to the heattreatment step to reduce particle growth during heat treatment. Thedeagglomeration step may involve mechanically milling the particles tobreak up agglomerates.

[0035] It should also be understood that not all methods of theinvention include an A/B ratio adjustment step and/or deagglomerationstep.

[0036] The methods of the present invention may involve adding one ormore dopant metal to the barium titanate-based composition. The dopantmetal(s) may be added either before or after heat treatment. In somemethods, one or more of the dopant metals may be added before heattreatment and one or more of the dopant may be added after heattreatment. The dopant metal(s) are selected to impart the resultingcomposition with the desired properties (e.g., electrical propertiessuch as dielectric constant and dissipation factor). Any dopant metalknown in the art may be used including Mg, Mn, W, Mo, V, Cr, Si, Y, Ho,Dy, Ce, Nb, Bi, Co, Ta, Zn, Al, Ca, Nd, and Sm. For some MLCCapplications, Y, Mg and Mn may be preferred dopant metals. Siliconcompounds (e.g., SiO₂, BaSiO₃) added as dopant metals may function assintering aids which reduce sintering temperatures.

[0037] In some cases, the dopant metals are coated on surfaces of thebarium titanate-based particles. The coating comprises at least one, butoftentimes more than one, dopant metal. The dopant metals in the coatingare typically in the form of metal oxides, hydroxides, or hydrousoxides. The form of the dopant metal compounds depends, in part, on theparticular dopant metal and the coating process.

[0038] The dopant metal coatings may be formed using any suitablecoating process. For example, the dopant metal coating may be formed byprecipitating the dopant metal compound(s) from an aqueous solution. Onesuitable precipitation technique involves forming a mixture of bariumtitanate-based particles and appropriate dopant metal solutions. A baseis added to the mixture to cause the dopant metal solutions toprecipitate on surfaces of the barium titanate-based particles. In somecases, the base may be added to the mixture in a manner that causes thedopant metals to sequentially precipitate onto surfaces of theparticles. The resulting particles are coated with respective layershaving different dopant metal compositions. This coating process andother suitable coating processes are described in U.S. patentapplication Ser. No. 10/194,936, filed Jul. 12, 2002, and entitled“Process for Coating Ceramic Particles and Compositions Formed From theSame,” by Venigalla et al, which is incorporated herein by reference inits entirety. Other suitable dopant coating processes have beendescribed, for example, in commonly-owned U.S. Pat. No. 6,268,054, whichis incorporated herein by reference in its entirety.

[0039] As described above, in some cases, the coating includes a seriesof chemically distinct layers. Each layer may comprise a differentdopant metal compound. It should be understood that the respectivelayers of the coating may not be entirely chemically distinct. That is,there may be a small percentage of other dopant metal compounds withineach layer and, in particular, near interfaces between adjacent layers.There may also be one or more layers that do not completely cover theparticle. These small amounts of inhomogeneity within the layers do notsignificantly effect the overall uniformity of the composition.

[0040] In some cases, the coatings are homogeneous with each dopantmetal distributed relatively uniformly throughout the coating. However,it should be understood that the homogeneous coatings may not include aperfectly homogeneous distribution of dopants.

[0041] The coatings may cover the entire particle surface, or only overa portion of the particle surface. In some embodiments, the coating mayhave a uniform thickness such that the thickness of the coating variesby less than 20% across the surface of an individual particle. In othercases, the thickness may vary by larger amounts. It is possible thatsome barium titanate-based particles may not be coated at all.

[0042] It should be understood that in some applications dopant metalsare not added to the barium titanate-based composition.

[0043] As noted above, the barium titanate-based particles may beprocessed to form dielectric layers in MLCC devices. In some processesfor forming MLCC devices, the particles may be mixed with a liquid(aqueous or non-aqueous) to form a slurry. Dispersants and/or bindersmay be added to the slurry to form a castable slip. The slip may be castto form a green layer. To form an MLCC, additional electrode layers andgreen layers may be deposited on top of one another. The structure issintered at elevated temperatures (e.g., between about 1200° C. andabout 1300° C.) to convert individual particles into a plurality ofgrains fused together to form a densifled dielectric layer. The grainare of a similar, or slightly greater, size than the particles. Theresulting MLCC includes alternating dielectric and electrode layers.

[0044] MLCCs produced according to the invention may include dielectriclayers having grain sizes equal, or similar, to the sizes of particlesfrom which the grains are formed. Similarly, MLCCs produced according tothe invention may include dielectric layers having tetragonalitiesequal, or similar, to the tetragonalities of the particles from whichthe grains are formed. For example, the MLCCs may include dielectriclayers having a grain size of equal to or less than about 0.3 micron anda tetragonality of equal to or greater than about 2.0, 2.5, or 3.0. Thedielectric layers may also have a particle size of equal to or less thanabout 0.2 micron or 0.1 micron, and a tetragonality of equal to orgreater than about 1.5, 2.0, 2.5, or 3.0. The particular combination ofgrain size and tetragonality may be selected based on the particularapplication and achieved using the methods described herein.

[0045] It should also be understood that the barium titanate-basedparticles of the present invention may be processed to form dielectricmaterials in other electronic devices, or otherwise as desired.

[0046] The following are examples that illustrates certain embodimentsof the invention. It should be understood that these are not limitingand do not illustrate all embodiments of the invention.

EXAMPLE 1

[0047] This example shows the effect of heat treatment temperature onthe tetragonality of barium titanate compositions.

[0048] Barium titanate particles were produced in a hydrothermalprocess. The barium source had a strontium concentration of greater than200 ppm. The average particle size of the barium titanate was about 0.15micron. The A/B ratio of the barium titanate composition was adjusted toa value of about 1.004. The barium titanate particles were divided intothree samples.

[0049] The first sample was heat treated at a temperature of about 1000°C. The second sample was heat treated at a temperature of about 960° C.The third sample was heat treated at a temperature of about 920° C. Eachsample had an average particle size of about 0.30 micron after heattreatment. The heat treatment times were selected to achieve the sameaverage particle size for each sample.

[0050] Each sample was analyzed using an x-ray powder diffractiontechnique to determine the tetragonality. The diffraction technique useda copper target at a voltage of 40 kV and a current of 35 mA to providea copper K-alpha monochromatic x-ray source, a sample rotation speed of60 rpm, a scan rate of 0.1°/minute and a scan range of 44°-46° 2-Theta.

[0051]FIG. 2 shows the x-ray diffraction pattern for the three samples.The tetragonalities were calculated from the diffraction pattern. Thesample heat treated at 1000° C. had a tetragonality of about 3.00. Thesample heat treated at 960° C. had a tetragonality of about 2.71. Thesample heat treated at 920° C. had a tetragonality of about 2.18.

[0052] The results show that heat treating barium titanate particlesaccording to a method of the present invention can produce bariumtitanate particles having high tetragonalities (e.g., greater than 2.0)and small particle sizes (e.g., 0.3 micron). The results also show thattetragonality increased with increasing heat treatment temperaturewithin the range considered.

EXAMPLE 2

[0053] This example shows the effect of strontium concentration on thetetragonality of barium titanate compositions.

[0054] Barium titanate particle samples were produced in three differenthydrothermal processes. In the first process, the barium source had astrontium concentration of 439 ppm. In the second process, the bariumsource had a strontium concentration of 387 ppm. In the third process,the barium source had a strontium concentration of 222 ppm. Thestrontium concentration was measured using an inductively coupled plasma(ICP) spectroscopy technique.

[0055] The average particle size for each sample was about 0.15 micron.The A/B ratios of the samples were not adjusted. Each sample was heattreated at 1000° C. The average particle size for each sample was about0.40 micron after heat treatment.

[0056] Each sample was analyzed using an x-ray powder diffractiontechnique to determine the tetragonality. The diffraction technique useda copper target at a voltage of 40 kV and a current of 35 mA to providea copper K-alpha monochromatic x-ray source, a sample rotation speed of60 rpm, a scan rate of 0.1°/minute and a scan range of 44°-46° 2-Theta.

[0057]FIG. 3 shows the x-ray diffraction pattern for the three samples.The tetragonalities were calculated from the diffraction pattern. Thesample formed using a barium source that had a strontium concentrationof 439 ppm had tetragonality of about 4.06. The sample formed using abarium source that had a strontium concentration of 387 ppm hadtetragonality of about 4.64. The sample formed using a barium sourcethat had a strontium concentration of 222 ppm had tetragonality of about5.53.

[0058] The results show that heat treating barium titanate particlesaccording to a method of the present invention can produce bariumtitanate particles having high tetragonalities (e.g., greater than 4.0)and small particle sizes (e.g., 0.4 micron). The results also show thattetragonality increased with decreasing strontium concentrations.

EXAMPLE 3

[0059] This example shows the effect of A/B ratio on particle sizegrowth during heat treatment of barium titanate compositions.

[0060] Barium titanate particles were produced in a hydrothermalprocess. The barium source had a strontium concentration of 50 ppm. Theaverage particle size of the barium titanate was about 0.060 micron.Three samples were produced by adjusting the A/B ratio to differentvalues. The first sample was washed with deionized water to reduce theA/B ratio to 0.983. The second sample was washed with deionized water toreduce the A/B ratio to 0.995. The third sample was processed to coatbarium carbonate on particle surfaces (using a precipitation techniqueinvolving adding barium hydroxide and ammonium carbonate to a slurry ofthe particles) to increase the A/B ratio to 1.013. The A/B ratio of afourth sample was not adjusted from 1.000.

[0061] The samples were heat treated at 1000° C. The average particlesize of each sample was measured. The sample having an A/B ratio of0.983 had an average particle size of about 0.18 micron. The samplehaving an A/B ratio of 0.995 had an average particle size of about 0.22micron. The sample having an A/B ratio of 1.013 had an average particlesize of about 0.24 micron. The sample having an A/B ratio that was notadjusted (1.000) had an average particle size of about 0.35 micron.

[0062] Each sample was analyzed using an x-ray powder diffractiontechnique to determine the tetragonality. The diffraction technique useda copper target at a voltage of 40 kV and a current of 35 mA to providea copper K-alpha monochromatic x-ray source, a sample rotation speed of60 rpm, a scan rate of 0.1°/minute and a scan range of 44°-46° 2-Theta.

[0063] The sample having an A/B ratio of 0.983 had a tetragonality ofabout 2.29. The sample having an A/B ratio of 0.995 had a tetragonalityof about 2.98. The sample having an A/B ratio of 1.013 had atetragonality of about 3.75. The sample having an A/B ratio that was notadjusted (1.000) had a tetragonality of about 5.66.

[0064] The results show that adjusting the A/B ratio of samples tovalues of greater than or less than 1.000 can reduce particle sizegrowth during heat treatment. The results also show that small bariumtitanate particles can be produced having high tetragonalities accordingto methods of the present invention.

[0065] Having thus described several aspects of at least one embodimentof this invention, it is to be appreciated various alterations,modifications, and improvements will readily occur to those skilled inthe art. Such alterations, modifications, and improvements are intendedto be part of this disclosure, and are intended to be within the spiritand scope of the invention. Accordingly, the foregoing description anddrawings are by way of example only.

What is claimed is:
 1. A composition comprising barium titanate-basedparticles having an average particle size of equal to or less than about0.3 micron and a tetragonality of equal to or greater than about 2.0. 2.The composition of claim 1 having an average particle size of equal toor less than about 0.2 micron.
 3. The composition of claim 1 having anaverage particle size of equal to or less than about 0.15 micron.
 4. Thecomposition of claim 1 having a tetragonality of equal to or greaterthan about 2.5.
 5. The composition of claim 1 having a tetragonality ofequal to or greater than about 3.0.
 6. The composition of claim 1,wherein the barium titanate-based particles are barium titanateparticles.
 7. The composition of claim 1, wherein the barium titanateparticles have a strontium and calcium concentration of less than about200 ppm.
 8. The composition of claim 1, wherein the bariumtitanate-based particles are hydrothermally-produced.
 9. The compositionof claim 1, wherein the barium titanate-based particles aresubstantially spherical.
 10. The composition of claim 1, furthercomprising at least one dopant metal.
 11. The composition of claim 10,wherein the barium titanate-based particles include a coating comprisingthe least one dopant metal.
 12. The composition of claim 1, wherein thecomposition has an A/B ratio of between about 1.000 and about 1.025. 13.The composition of claim 1, wherein the composition has an A/B ratio ofbetween about 0.975 and about 1.000.
 14. A composition comprising bariumtitanate-based particles having an average particle size of equal to orless than about 0.15 micron and a tetragonality of equal to or greaterthan about 1.5.
 15. The composition of claim 14 having a tetragonalityof equal to or greater than about 2.0.
 16. The composition of claim 14,wherein the barium titanate-based particles are barium titanateparticles.
 17. The composition of claim 14, wherein the barium titanateparticles have a strontium and calcium concentration of less than about200 ppm.
 18. The composition of claim 14, wherein the bariumtitanate-based particles are hydrothermally-produced.
 19. Thecomposition of claim 14, wherein the barium titanate-based particles aresubstantially spherical.
 20. The composition of claim 14, furthercomprising at least one dopant metal.
 21. The composition of claim 20,wherein the barium titanate-based particles include a coating comprisingthe least one dopant metal.
 22. The composition of claim 14, wherein thecomposition has an A/B ratio of between about 1.000 and about 1.025. 23.The composition of claim 14, wherein the composition has an A/B ratio ofbetween about 0.975 and about 1.000.
 24. A multilayer ceramic capacitorcomprising: an electrode layer; and a dielectric layer formed on theelectrode layer, the dielectric layer including a plurality of grains,the grains comprising a barium titanate-based material and having anaverage grain size of equal to or less than about 0.3 micron and atetragonality of equal to or greater than about 2.0.
 25. The multilayerceramic capacitor of claim 24 having an average grain size of equal toor less than about 0.2 micron.
 26. The multilayer ceramic capacitor ofclaim 24 having an average grain size of equal to or less than about0.15 micron.
 27. The multilayer ceramic capacitor of claim 24 having atetragonality of equal to or greater than about 2.5.
 28. The multilayerceramic capacitor of claim 24 having a tetragonality of equal to orgreater than about 3.0.
 29. The multilayer ceramic capacitor of claim24, wherein the grains comprise barium titanate.
 30. A methodcomprising: mixing a barium source and a titanium source to form areaction mixture, the barium source having a concentration of strontiumand calcium of less than about 200 ppm; maintaining the reaction mixtureat an elevated temperature while the barium source reacts with thetitanium source to produce barium titanate-based particles; and heattreating the barium titanate-based particles at a temperature of betweenabout 850° C. and about 1150°C.
 31. The method of claim 30, wherein thebarium titanate-based particles have an average particle size of equalto or less than about 0.3 micron.
 32. The method of claim 30, whereinthe barium titanate-based particles have a tetragonality of equal to orgreater than about 2.0.
 33. The method of claim 30, comprising heattreating the barium titanate-based particles at a temperature of betweenabout 950° C. and about 1050° C.
 34. The method of claim 30, furthercomprising adjusting an A/B ratio of the barium titanate-based particlesprior to the heat treating step to a value between about 0.975 and 1.000or between about 1.000 and about 1.025.
 35. The method of claim 34,comprising adjusting the A/B ratio to a value between about 0.982 andabout 0.988 or between about 1.008 and about 1.012 prior to the heattreating step.
 36. The method of claim 30, further comprising processingthe barium titanate-based particles to form a dielectric layer of amultilayer ceramic capacitor.
 37. The method of claim 30, wherein thebarium titanate-based particles are barium titanate particles.
 38. Themethod of claim 30, further comprising coating the barium titanate-basedparticles with at least one dopant metal.
 39. A method comprising:mixing a barium source and a titanium source to form a reaction mixture;maintaining the reaction mixture at an elevated temperature while thebarium source reacts with the titanium source to produce bariumtitanate-based particles; adjusting an A/B ratio of the bariumtitanate-based particles; and heat treating the barium titanate-basedparticles, after A/B ratio adjustment, at a temperature of between about850° C. and about 1150° C.
 40. The method of claim 39, wherein thebarium source has a concentration of strontium and calcium of less thanabout 200 ppm.
 41. The method of claim 39, wherein the bariumtitanate-based particles have an average particle size of equal to orless than about 0.3 micron.
 42. The method of claim 39, wherein thebarium titanate-based particles have a tetragonality of equal to orgreater than about 2.0.
 43. The method of claim 39, comprising heattreating the barium titanate-based particles at a temperature of betweenabout 950° C. and about 1050° C.
 44. The method of claim 39, furthercomprising adjusting the A/B ratio of the barium titanate-basedparticles prior to the heat treating step to a value between about 0.975and 1.000 or between about 1.000 and about 1.025.
 45. The method ofclaim 39, comprising adjusting the A/B ratio to a value between about0.982 and about 0.988 or between about 1.008 and about 1.012 prior tothe heat treating step.
 46. The method of claim 39, further comprisingprocessing the barium titanate-based particles to form a dielectriclayer of an MLCC after the heat treating step.
 47. The method of claim39, wherein the barium titanate-based particles are barium titanateparticles.
 48. The method of claim 39, further comprising coating thebarium titanate-based particles with at least one dopant metal.